This invention relates to a compressed-gas circuit breaker having two contact members which move relative to one another, a nozzle which is made of dielectric material and is attached to a first of the two contacts members and in which compressed gas is caused to flow through the constriction of the nozzle from a compression space into an expansion space, the expanding gas being used for extinguishing the arc which occurs when the circuit breaker is operated.
A circuit breaker of this type has been disclosed in German Auslegeschrift No. 2,039,240. In this circuit breaker, the nozzle, which is made of dielectric material and is attached to the first contact member, possesses a graphite part in the region of the nozzle constriction. This graphite part is designed in the shape of an annulus and is electrically connected, via a current collector, to the main contact of the second contact member. This main contact is designed as a tubular sleeve. The graphite part is provided in order to receive the root of the arc which is struck between the contact members as the circuit is broken, this arc-root being located on the second contact member and its reception, by the graphite part, occurring as soon as the distance between the contact members reaches a predetermined value. Since the arc thereafter burns between the nozzle and the contact member attached thereto, the distance between the roots of the arc remains constant, the intention being, by this means, to avoid unnecessary elongation of the arc and to increase the circuit-breaking capacity.
In the case of this arrangement, however, the electric field, which is generated at the breaker-gap by the transient recovery voltage, after the arc has been extinguished in the zero-current condition, is forced into that portion of the compression space which is situated between the nozzle and the first contact member and which, despite being subject to a high pressure nevertheless has a narrow configuration and is subject to high thermal loading. Moreover, the extension of the arc on to the nozzle gives rise to an extremely high surface temperature. Both phenomena lead to impairment of the dielectric hardening of the breaker-gap.
The object of the invention is to produce a circuit breaker of this generic type, in which the reception of an arc-root on the nozzle surface is avoided, and in which the nozzle can be exposed to a high thermal loading in the region of its constriction, without the occurrence of instances of arcing-over via the thermally loaded nozzle surface when high voltages are applied to the breaker-gap.
This object is achieved by providing at least one annular insert at the nozzle constriction. This insert is electrically isolated with respect to the two contact members and exhibits a first capacitance with respect to the first contact member and a second capacitance with respect to the second contact member. The magnitudes of the capacitance are chosen, by suitable arrangement and dimensioning of the insert, so that the electric field, which is formed when a voltage exists between the contact members, is displaced in the region of the nozzle constriction, at least partially from the surface of the nozzle into the compression space and into the expansion space. The circuit breaker according to the invention is distinguished by the fact that, in contrast to comparable known breakers, the circuit breaking capacity is increased by simple means; that is, the electric field generated by the transient recovery voltage is displaced from the region of the nozzle constriction, at least partially into the spaces situated in front of and behind the nozzle constriction, as a result of which the stress-loading in the constriction region of the nozzle is reduced following the extinguishing of the arc.
It is additionally possible, depending on the design configuration of the compressed-gas circuit breaker, to relocate a greater or lesser portion of the electric field into the space in front of or behind the nozzle constriction.
If the insert of the compressed-gas circuit breaker according to the invention is configured so that the insert is comprised of a material having a dielectric constant greater than 5 extension of the arc at the nozzle is avoided in a particularly effective manner.
It is advantageous, in order also to be able to use inserts of a material which does not exhibit an overly high burn-off resistance with respect to the arc associated with the breaking of the circuit, to embed the insert in the dielectric material of the nozzle. This embedding may be effected so that the insert contains diffusely distributed powder which is comprised of a conductive material or has a dielectric constant greater than 5. Thus, local field-peaks on the surface of the nozzle (which is made of dielectric material) are avoided. Direct contact between the arc and the embedded material is also avoided while, a smooth transition of the electric field intensity is achieved at the ends of the region of the nozzle constriction.